Change header accent base color to white in Fast Templates

[MarcSkovMadsen]

Fixes #2610

Looks like

when running

import panel as pn

pn.extension()

template=pn.template.FastListTemplate(title="App with Menu", header_accent_base_color="white")

ACCENT_BASE_COLOR = template.accent_base_color

COLLAPSED_ICON = f"""<svg style="stroke: { ACCENT_BASE_COLOR }" width="18" height="18" viewBox="0 0 18 18" fill="none" xmlns="http://www.w3.org/2000/svg" slot="collapsed-icon">
<path d="M15.2222 1H2.77778C1.79594 1 1 1.79594 1 2.77778V15.2222C1 16.2041 1.79594 17 2.77778 17H15.2222C16.2041 17 17 16.2041 17 15.2222V2.77778C17 1.79594 16.2041 1 15.2222 1Z" stroke-linecap="round" stroke-linejoin="round"></path>
<path d="M9 5.44446V12.5556" stroke-linecap="round" stroke-linejoin="round"></path>
<path d="M5.44446 9H12.5556" stroke-linecap="round" stroke-linejoin="round"></path>
</svg>"""

EXPANDED_ICON = f"""<svg style="stroke: { ACCENT_BASE_COLOR }" width="18" height="18" viewBox="0 0 18 18" fill="none" xmlns="http://www.w3.org/2000/svg" slot="expanded-icon">
<path d="M15.2222 1H2.77778C1.79594 1 1 1.79594 1 2.77778V15.2222C1 16.2041 1.79594 17 2.77778 17H15.2222C16.2041 17 17 16.2041 17 15.2222V2.77778C17 1.79594 16.2041 1 15.2222 1Z" stroke-linecap="round" stroke-linejoin="round"></path>
<path d="M5.44446 9H12.5556" stroke-linecap="round" stroke-linejoin="round"></path>
</svg>
"""

MENU_HTML=f"""
<fast-accordion id="menu">
    <fast-accordion-item slot="item" expanded>
        <h3 slot="heading" style="margin:0px">Introduction</h3>{ COLLAPSED_ICON }{ EXPANDED_ICON }
        <ul>
            <li><a href="introduction">Introduction</a></li>
        </ul>
    </fast-accordion-item>
    <fast-accordion-item slot="item" expanded>
        <h3 slot="heading" style="margin:0px">Example Apps</h3>{ COLLAPSED_ICON }{ EXPANDED_ICON }
        <ul>
            <li class="menu-item-active"><a href="/">ML Explainability</a></li>
            <li><a href="/">Plots and Tables with Events</a></li>
            <li><a href="/">Cross Filtering</a></li>
            <li><a href="/">Streaming Data</a></li>
        </ul>
    </fast-accordion-item>
</fast-according>
"""

template.sidebar_footer=MENU_HTML

template.servable()

[codecov]

Codecov Report

Merging #2611 (5a928d2) into master (b220588) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2611   +/-   ##
=======================================
  Coverage   83.09%   83.09%           
=======================================
  Files         184      184           
  Lines       23295    23295           
=======================================
  Hits        19356    19356           
  Misses       3939     3939           

Impacted Files	Coverage Δ
panel/template/fast/theme.py	97.72% <100.00%> (ø)

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